Light emitting efficiency of a light emitting diode is dominated by its internal quantum efficiency and light extraction efficiency. Internal quantum efficiency relates to the light generated from an active layer. Light extraction efficiency is the ability that the light from the active layer emits to medium surrounded. With development of epitaxy technology, internal quantum efficiency can be up to 80%. However, light extraction efficiency is still low. For example, refraction index of GaN series materials is about 2.5. The air around them has refraction index of 1. Due to total reflection, the light extraction efficiency in the interface is only 10˜12%.
In order to enhance light extraction efficiency, randomly etched cavities are formed on the surface of a transparent conductive layer. Therefore, most light beams from the active layer can emit out of the light emitting diode without being reflected. Alternatively, roughening a p-type layer can also achieve the same effect.
Generally, thickness of the top epitaxial structure of a GaN or AlGaInP series light emitting diode for generating red or yellow beams is larger than 5 μm. Hence, plasma or chemical etching can be applied to make the cavities or a two dimensional pattern. However, for other light emitting diodes generating blue, green or UV light beams, the top epitaxial structure is very thin (about 0.2 μm). If the external quantum efficiency needs to be improved to enhance light extraction efficiency, depth of the cavities should be at least 0.2 μm. Therefore, traditional surface roughening methods are not proper for this purpose.
Additionally, traditional etching roughening methods usually use photoresist as a mask. Due to low etching selectivity, these methods can not be used to make a desired etching depth. Not to mention a deeper depth. Therefore, it is not easy to pattern or roughen the light emitting diode. Moreover, while metallic materials, such as nickel, are used as a hot mask, a photoresist needs to be in advance spread on the light emitting diode before the hot mask is placed, such that manufacturing complexity and cost are increased.
Traditionally, surface roughening methods may create patterns having protrusions with distance therebetween over 2˜3 um. This is disadvantageous for forming delicate patterns to improve light extraction efficiency. Moreover, traditional etching methods can only be used to roughen top surface of the light emitting diode and are not able to roughen the edges thereof.
In order to overcome the problems mentioned above, an improved method is disclosed in U.S. Pat. No. 6,551,936. Please see FIG. 1. It is for etching a pattern in a semiconductor material based on the formation of an InP grating mask on the semiconductor material. The formation of the InP grating mask involves the formation of a multi-layered structure on the semiconductor material with an etch-stop layer between two InP layers. A photoresist grating mask corresponding to the pattern to be etched in the semiconductor material is then formed on the top InP layer. Subsequently, a non-selective etch is used to penetrate the top InP layer, the etch-stop layer, and the lower InP layer. A suitable stripping solvent is then used to remove the photoresist followed by a selective etch to clear the remaining exposed InP material, remove contaminated material and to expose the underlying semiconductor material in accordance with the pattern to be etched. Additional masking beyond the InP mask is, therefore, not required. The exposed semiconductor material is then etched such that the pattern is transferred to the semiconductor material.
Although the above invention solves most of the problems, the pattern formed is still restricted. Not any desirable pattern for enhancing light extraction efficiency can be controlled and achieved.